1. presents
a production
Everything Electric
Episode 3
Mainly Mains
2. Mainly Mains
The Mains is the electric power
supply to a house or workplace.
bestofyoutube.com
It is alternating current (AC) which means the electricity
repeatedly changes direction in the circuit (50 times per
second). Batteries provide direct current (DC); always in
the same direction.
3. The hazards of mains electricity
Overloaded socket Fire Shock
Images: Health and Safety signs
Image: Citizencorps.gov
Water near sockets
Images: OCAL @ clker.com
Image: Childsafetyaustralia
Frayed cables Metals in appliances Image: Safework SA
4. Electrical safety
Image: aka @ wikipedia
Double
insulation
Fuse
Image: Jared C Bennett @ wikipedia
Earth
wire
Circuit
breaker
Image: wn.com Image: Aussiedingo @ flickr.com
5. Fuses and circuit breakers
When the current is too high,
they melt or break the circuit.
Image: wn.com
Q: A heater has a maximum
current of 10 A; what is the
appropriate circuit breaker or
fuse for safety?
Choose from 8, 16, 20 or 32 A
HINT: the ‘rating’ must be
greater than the max.
8 A - too low current, but not by too much.
16 A - this one
20 A, 32 A - too high; maybe good for an electric oven.
6. Earth wire
Normally there is no
current in the earth wire.
A fault can cause the
metal case to be live.
The earth conducts the
current away from the case.
The current in the earth
wire is often high enough
to melt the fuse.
Images: OUPChina
7. Double insulation
Image: Jared C Bennett @ wikipedia
Many appliances have an insulating cover (eg plastic);
this is called double insulation since the wires inside
are also insulated. So a fault could not give the user a
shock from the case. No earth wire is necessary.
8. Electrical heating
Components transfer some or all of the electrical energy
to heat energy which then warms the surroundings.
Energy transfer
by radiation
Image: opencage.info
Image: Fireflyco.com
Energy transfer Image: Pokayoke @ wikispaces
by conduction Energy transfer
(and convection by convection
in the water)
9. Calculating electrical power
Power is the rate of transfer of energy to other forms
P
I xV
Power, P is measured in watts (W)
Voltage,V is measured in volts (V)
Current, I is measured in amps (A)
To calculate power in watts, multiply volts times amps.
Look at the label on a computer or kettle etc.
10. Calculating electrical energy
Energy, E is measured in joules (J)
Voltage,V is measured in volts (V)
Current, I is measured in amps (A)
Time, t is measured in seconds (s)
Power, P is measured in watts (W)
Reminder: Power = Energy/ Time
P E E
I xV Pxt
VxIxt
11. Power calculations
A movie projector converts 4.32 MJ showing
a 3 hour movie. What is its power?
A kettle rated at 1.5 kW boils water for 2
minutes. How much energy is converted?
What current passes through a 12 V bulb
which is rated at 3.0 W?
How much energy is converted in 30 min by
a laptop whose supply voltage is 12 V and
which takes a current of 1.0A?
ANSWERS ON NEXT BUT ONE PAGE
12. Extended Power calculations
More than IGCSE demands: just for fun
A van de Graaf generator develops a PD of 200 kV. It
sparks with a current of 2.0 mA in 2.0 ms.
A: How much energy is converted in joules?
B: How much electric charge is discharged in coulombs?
C: The charge on one electron is 1.6 x 10-19 C. How
many electrons are discharged?
D: One electron-volt (eV) is a unit of energy. An electron
accelerated through 200 kV converts 200 keV. The Large
Hadron Collider gives protons energies of 4 TeV (2012).
How many times more energetic is this than the vdGG?
ANSWERS ON NEXT PAGE
14. a production
MUCH MORE AT
http://nothingnerdy.wikispaces.com
http://nothingnerdy.wikispaces.com/MAINS+ELECTRICITY
Notas del editor
Welcome to nothingnerdy’s presentation on Everything Electric. Part 3: Mainly Mains\n
\n
The dangers of mains electricity are fire and electric shock. The following are hazards for one or both of these reasons: Damaged plug; overloaded socket; frayed cable; metal objects in the wrong place; water around sockets.\n
Different devices are used to make us safe from electrical hazards. Wires are insulated and cables double insulated as in the photo above. A fuse is a weak point in a live wire which will melt if the current is too high. A circuit breaker also breaks the connection when the current is too high. An earth wire connects the metal case of a device to the ground so that if the case goes accidentally live the current flows to earth and blows the fuse of circuit breaker. See the earth symbol.\n
Different devices are used to make us safe from electrical hazards. Wires are insulated and cables double insulated as in the photo above. A fuse is a weak point in a live wire which will melt if the current is too high. A circuit breaker also breaks the connection when the current is too high. An earth wire connects the metal case of a device to the ground so that if the case goes accidentally live the current flows to earth and blows the fuse of circuit breaker. See the earth symbol.\n
If a loose live wire in a device touches the metal case, you can get a nasty shock if you touch the metal. This is why many devices have an earth wire connecting the case to the earth terminal in the house. Now if there is a fault, the current runs straight to the earth. This can cause a high current which blows the fuse preventing shock or fire.\n
Different devices are used to make us safe from electrical hazards. Wires are insulated and cables double insulated as in the photo above. A fuse is a weak point in a live wire which will melt if the current is too high. A circuit breaker also breaks the connection when the current is too high. An earth wire connects the metal case of a device to the ground so that if the case goes accidentally live the current flows to earth and blows the fuse of circuit breaker. See the earth symbol.\n
When a current flows in a resistor, the electrical energy converts to heat and the resistor becomes hot. We can use this process in heating devices. A kettle warms liquid with an electric element; a bar heater radiates heat into the room; a fan heater circulates hot air in the room.\n
Power in watts measures how much energy in joules is converted per second. The formula is P=VI. Many devices have a label showing their operating voltage and current. If you multiply these two values, you can calculate the electrical power.\n
We know that Power=V times I and also that Energy=Power times time. Combining those two formulas gives us E=VIt\n
We know that Power=V times I and also that Energy=Power times time. Combining those two formulas gives us E=VIt\n
We know that Power=V times I and also that Energy=Power times time. Combining those two formulas gives us E=VIt\n
We know that Power=V times I and also that Energy=Power times time. Combining those two formulas gives us E=VIt\n
